- The Washington Times - Saturday, March 23, 2002

ASSOCIATED PRESS
An unusually warm period a millennium ago may have been part of a natural planetary cycle, researchers say in a study of tree rings that scrutinizes the link between human activity and climate change.
The study, appearing yesterday in the journal Science, analyzed ancient tree rings from 14 sites on three continents in the Northern Hemisphere and concluded that temperatures in an era known as the Medieval Warm Period about 800 to 1,000 years ago closely matched the warming trend of the 20th century.
In recent years, many climate scientists have said an unprecedented warming spell that began last century is caused by the "greenhouse" effect. The greenhouse effect is blamed on an increase in the atmosphere of gases, principally carbon dioxide, from the burning of fossil fuels, which trap heat just as glass panes do in a greenhouse.
The tree-ring study gives another perspective on Earth's natural cycles, said Edward Cook of the Lamont-Doherty Earth Observatory in Palisades, N.Y. Mr. Cook is co-author of the study with Jan Esper and Fritz Schweingruber of the Swiss Federal Research Institute.
Mr. Cook said the study shows the Earth to be "capable of rapid changes and long periods of above-average warmth on its own, without greenhouse warming."
"We don't use this as a refutation of greenhouse warming," Mr. Cook said. "But it does show that there are processes within the Earth's natural climate system that produce large changes that might be viewed as comparable to what we have seen in the 20th century."
Mr. Cook said the study found that, based on the growth of rings in the trunks of trees that lived hundreds of years ago, the temperatures during the Medieval Warm Period were about equal to the warming trend that started in the 20th century.
"Greenhouse gases were not a factor back in the Medieval Warm Period," Mr. Cook said.
The Intergovernmental Panel on Climate Change, an international group, has predicted that the current warming trend will continue deep into the 21st century, with average temperatures rising by from 2.5 to 10 degrees. Based on this prediction, there have been international proposals for systematic reductions in the burning of fossil fuels. The proposal has been resisted by the United States, particularly the Bush administration.
Mr. Cook said data used in the climate-change panel's calculation is based on a model that compared the preindustrial-age climate with the climate of the 20th century. The model did not include a Medieval Warm Period. Including data from that era could change the calculations, Mr. Cook said.
"The Medieval Warm Period is in some sense comparable up to 1990 in the 20th century," said Mr. Cook. "But that does not say that the 20th century hasn't been perturbed by greenhouse gases. The real challenge is to factor out the natural variability from" man-made causes of global warming.
Mr. Cook said the panel's temperature-warming prediction could be correct. Based on the new tree-ring data, however, he said the warming could be in the lower part of the temperature range forecast by the group.


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